Why are some terms grayed out in the tree window?
If a word appears in gray in either your search results or the tree window, it means that there are no results in our database for that combination. You might want to then choose a “parent” of this term (i.e., a more general category). The terms in the tree represent a taxonomy, which goes from the more general concepts to the more specific, so at any time you can choose a general concept to get more results.
